Tachycardia is an abnormal heart rhythm characterized by rapid activation of one or more chambers of the heart of a patient. Tachycardia is often qualified by the locus of origin: a tachycardia that originates in the ventricles of the heart is called a ventricular tachycardia (VT) and a tachycardia that originates in the atria of the heart is called an atrial tachycardia (AT) or a supraventricular tachycardia (SVT). Some VTs, if untreated, may accelerate into ventricular fibrillation, in which the pumping ability of the heart is seriously impaired.
In a single patient, two episodes of tachycardia may be different. The causes for these differences could be many, such as disease progression, changes in medication, presence of multiple reentrant circuits and differences in site of origin. Hence, a single patient may experience distinct VTs or ATs over time.
Some tachycardias respond well to medication, and others may be treated with surgery such as radio frequency (RF) ablation. In some patients, VT or AT may respond well to antitachycardia pacing (ATP), in which small electric stimulations from an implantable pulse generator (IPG) in an implantable medical device (IMD) disrupt the propagation of electrical signals that cause the tachycardia. The IMD may be programmed to administer several forms of ATP therapies, and may apply one ATP therapy after another until the tachycardia terminates.
In some circumstances, a tachycardia may fail to terminate in response to ATP, and therefore certain IMDs include the capability of delivering a higher energy cardioversion shock to terminate the tachycardia. In addition, some IMDs may be programmed to administer several cardioversion shocks of progressively increasing energy. Cardioversion therapies tend to be painful compared with ATP therapies, which tend to go unnoticed by the patient. As a result, since ATP therapies may be just as effective for a given episode as cardioversion shock therapies, some patients are treated with one or more ATP therapies prior to being treated with one or more cardioversion therapies in an attempt to avoid the discomfort associated with cardioversion.